1. Field of the Invention
The present invention relates generally to the field of electrochemical cells and to the processing of electrodes for such cells. More particularly, the invention addresses improved binder materials for use in improving cathode adhesion to thin metal current collectors.
2. Related Art
Cathode processing is a critical element in the development of any battery technology as the physical properties of finished cathodes determine, to a large extent, the performance capabilities of the resulting cells and batteries. Cathode processing also represents a significant portion of the total cell and/or battery cost. This adds importance to the necessity of the development of low-cost, production-oriented manufacturing techniques in order to make products practical with any applicable electrochemical technology.
Binders are an essential component of any cathode technology. The purpose of the binder is to hold the electrode materials together, giving the structure ruggedness and the ability to be handled easily without damage. It is also often highly desirable that the binder possess elastomeric qualities in order to produce flexible cathodes suitable for spirally-wound electrode configurations. These properties must be achieved, however, while porosity and electrical conductivity are still maintained throughout the bulk of the electrode. Of course, it is also essential that the binder material be inert to the electrolyte solution employed and be electrochemically inactive at the normal operating potentials of the cell. To date, Teflon (Trademark of the duPont Company, Wilmington Del.) or polytetrafluoroethylene (PTFE) has been the predominant binder material employed in non-aqueous, specifically lithium, cells.
For the most part, Teflon-bonded electrodes have been found to offer excellent physical and performance (electrochemical) characteristics. One major drawback which has been encountered with Teflon-bonded electrodes, however, is the fact that Teflon is highly reactive with lithium metal and this can cause safety problems if such cells are over discharged to the point where lithium is electrochemically deposited at the cathode.
In addition, more recent work with regard to rechargeable lithium systems involves the development of thin cathodes. The purpose of this is to increase the electrode surface area that can be packaged within a given cell volume and thus reduce the local current densities to the lowest possible levels. It is expected that this thin cathode technological approach will significantly improve the operating efficiencies and cycle life capabilities of such cells.
One method to achieve very thin electrodes is to coat thin layers of the active cathode material onto metal foil substrates. This has been done with slurry-coating processes including dip coating, spraying, or painting, all of which are production-oriented and potential low-cost techniques. For this approach to be successful, of course, good adhesion of the cathode material to the substrate must be achieved.
Here the traditional Teflon material again falls short. Although PTFE is a good material for binding the electrode material itself together, it is not particularly well suited to laminating powder cathode materials to metal foil substrates.
For these and other reasons a need has existed to develop alternate binders which are chemically inert to alkali metals or other reactive species in the cell and which, at the same time, possess good adhesive properties with regard to binding the electrode material together and to metal foil substrates. This has led to much work in the field to find suitable replacement binding materials.